Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs capable of operation across the visible spectrum include group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus, also referred to as III-phosphide materials. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide by metal organic chemical vapor deposition (MOCVD) molecular beam epitaxy (MBE) or other epitaxial techniques. These LED device structures can also be transferred to a transparent substrate by wafer bonding. Often, an n-type layer (or layers) is deposited on the substrate, then an active region is deposited on the n-type layers, then a p-type layer (or layers) is deposited on the active region. The order of the layers may be reversed such that the p-type layers are adjacent to the substrate by either epitaxial growth or wafer bonding.
FIG. 1 illustrates a cross-sectional view of a conventional light emitting diode (LED) 10. As shown in FIG. 1, one or more p type layers are formed over a substrate 12. By way of example, a p-AlInP layer 16 may be formed over a p doped region 14 of a GaP substrate 10 by wafer bonding, and p-contacts 18 are formed on the p doped region 14. An active region 20 is formed over the p type layer 16 and an n type layer 22, e.g., an n-AlInP Layer, is formed over the active region 20. An n contact 24 is formed over the n type layer 22, but the contact area is minimized in order to increase the area of the reflective mirror 26 area for better light extraction through the substrate 12. Thus, the LED 10 can be used in a flip chip configuration with the p-contacts 18 and n-contacts 24 formed on the same side of the device when flip-chipped on a submount and where the light is extracted through the substrate 12, which is the top of the device.
The design scheme of the flip chip LED 10 forces lateral current injection, which results in current crowding under the n-contact 24 and near the p contact area 18 as illustrated by the arrows in FIG. 1. The current crowding results in non-uniform current injection as well as high series resistance and high forward voltage Vf compared to vertical injection LEDs.
One manner of solving the non-uniform current injection problem in the n-side is to use full sheet n-metal contact. However, because the n-metal contact has to be annealed at high temperature, e.g., greater than 420° C., to achieve a good ohmic contact, the metal surface is rough. As a result, the reflectively of the full sheet n-metal contact is poor and thus, decreases light extraction.
Thus, it is highly desirable to improve the contacts used with LEDs reduce the non-uniform current injection problem without decreasing light extraction.